powerpc, 8xx: remove support for 8xx

There was for long time no activity in the 8xx area.
We need to go further and convert to Kconfig, but it
turned out, nobody is interested anymore in 8xx,
so remove it (with a heavy heart, knowing that I remove
here the root of U-Boot).

Signed-off-by: Heiko Schocher <hs@denx.de>